Method of preparing organic isocyanates in the presence of molecular sieves

ABSTRACT

An improved process is disclosed for the phosgenation of aliphatic and aromatic amines to obtain the corresponding organic isocyanates. The improvement comprises carrying out the known phosgenation procedure in the presence of crystalline zeolitic molecular sieves. The organic isocyanate products of the improved process show lighter coloration and improved color stability in comparison to isocyanates obtained by the prior known phosgenation processes.

United States Patent Thompson [45] Oct. 10,1972

[54] METHOD OF PREPARING ORGANIC ISOCYANATES IN THE PRESENCE OFMOLECULAR SIEVES [72] Inventor: Edward J. Thompson, Watertown,

Conn.

[73] Assignee: The Upjohn Company, Kalamazoo,

Mich.

[22] Filed: July 30, 1969 [21] Appl. No.: 846,241

[52] US. Cl. ..260/453 PH, 252/455 Z, 252/463 3,211,631 10/1965 Fuchs..260/453 X 3,330,849 7/1967 Ulrich ..260/453 3 ,449,395 6/1969 Majewskiet a1 ..260/45 3 2,882,244 4/1959 Milton ..252/455 3,412,134 1l/l968Jones ..252/455 X 2,885,420 5/1959 Spiegler ..260/453 2,950,307 8/1960France et a1 ..260/453 Primary ExaminerFloyd D. Higel Att0rneyEugene O.Retter and Denis A. Firth [5 7] ABSTRACT An improved process isdisclosed for the phosgenation of aliphatic and aromatic amines toobtain the corresponding organic isocyanates. The improvement comprisescarrying out the known phosgenation procedure in the presence ofcrystalline zeolitic molecular sieves. The organic isocyanate productsof the improved process show lighter coloration and improved colorstability in comparison to isocyanates obtained by the prior knownphosgenation processes.

4 Claims, No Drawings METHOD OF PREPARING ORGANIC ISOCYANATES IN THEPRESENCE OF MOLECULAR SIEVES BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to an improved process for thepreparation of aliphatic and aromatic isocyanates by phosgenation of thecorresponding amines and is more particularly concerned with a processfor preparing organic isocyanates of improved color and forcolor-stabilizing of organic isocyanates by carrying out the preparationof the isocyanate in the presence of molecular sieves.

2. Description of the Prior Art Aliphatic and aromatic isocyanatesincluding both mono and polyfunctional types, hereinafter termedisocyanates, are prepared in a variety of ways. The most widely usedcommercial methods are by phosgenation of the corresponding amine, usingtechniques which are for the most part well-known. The isocyanates aredesirable compounds for use in the preparation of polymer surfacecoatings and the like, but selection of particular isocyanates to be soemployed is limited to those which are themselves colorless, andtherefore less apt to impart undesired color initially to the polymerswhen formed. The limitation is a severe one, since isocyanatesparticularly desirable on an economic basis tend to be colored, and alarge number of isocyanates otherwise suitable for preparing polymercoatings, tend to discolor upon storage. This discoloration, Le: achange to yellow or brown may proceed rapidly, even though air,moisture, and light are excluded from contact with the isocyanates.Since the precise causes of color formation and discoloration areunknown, attempts to obtain colorless isocyanates have been mainlyconcerned with prevention of discoloration in those isocyanates whichare colorless when made. One approach requires the addition ofstabilizers as for example organic sulfonyl isocyanate [U.S. Pat. No.3,330,849 (Ulrich)]. Generally, the use of additives is not entirelysatisfactory, since some additives interfere with, or adversely affect,particular uses of the isocyanate. The use of active charcoal oractivated earth filters, standard fluid decolorizing methods, has beenconsidered but is not entirely satisfactory or efficient, yieldingvariable results while adding considerable cost to the product. Theideal then, is a standard commercial process, modified in a minor way,to yield colorless isocyanates which are otherwise normally colored, andto have these colorless isocyanates remain color stable under normalstorage conditions for extended periods of time. Surprisingly, I havefound this can be achieved by the process to be described hereinafter.

SUMMARYOF THE INVENTION This invention is an improvement in the methodfor producing an organic isocyanate which is normally colored, by thereaction of phosgene with a member selected from the group consisting ofthe corresponding amine and salt thereof. The improvement comprisescarrying out the reaction in the presence of a molecular sieve. Theisocyanates produced by this improved process are lighter in color thenthose isocyanates prepared without including molecular sieves in thereaction. The invention is also concerned with the products obtained byuse of the improved process.

DETAILED DESCRIPTION OF THE INVENTION The conversion of primary aminesand amine salts to corresponding aliphatic and aromatic isocyanates byphosgenation is amply described in the literature and many variations ofreaction conditions with attendant variations of product are well-known(Polyurethanes:

Chemistry and Technology, Part I, Saunders and Frisch, published bylnterscience Pub., N.Y., 1962, pp. 18-24). Thus, depending upon theamine starting materials and reaction conditions, there are producedaliphatic and aromatic isocyanates which are mono, di andpolyisocyanates and mixtures thereof. Illustrative of typical aminesemployed are methylene bis(4,4'- cyclohexyl amine), 1,6-hexamethylenediamine, 2,4- diaminotoluene, aniline, 2,6-tolylene diamine, 2,4,6-tolylene triamine, toluidine, benzidine, naphthylamine, hexylamine,octylamine, dodecylamine, octadecylamine, tetramethylenediamine,octamethylene diamine, cyclohexylamine, phenylethylamine,phenylhexylamine, naphthyldodecylamine, methylene bis(4- phenylamine),1,5-naphthylene diamine, chloroaniline, chlorohexylamine and the likeand mixtures thereof. Amine salts employed are preferably thehydrochlorides and hydrobromides corresponding to the amines. Many ofthe crude aliphatic isocyanates, produced by these methods are colored,ranging in color from light yellow to dark brown.

Illustrative of colored isocyanates which may be prepared colorless orof lighter coloration, and of those isocyanates which have better colorstability when prepared by the process of this invention arearyl'isocyanates of six to seven carbon atoms, inclusive, such asphenylisocyanate, and tolyl isocyanate; chlorophenyl isocyanate, arylenediisocyanate of six to 12 carbon atoms, inclusive, such as meta or paraphenylene diisocyanates, m-xylylene diisocyanate, p-xylylenediisocyanate, naphthalene-l ,5-diisocyanate, diphenyl-4,4- diisocyanate,and tolylene diisocyanate, including mixtures of the 2,4- and2,6-isomers thereof; diphenylmethane-4,4'-diisocyanate and mixturesthereof containing polyisocyanates of higher functionality, 3-methyldiphenylmethane-4,4'-diisocyanate,chlorophenylene-2,4-diisocyanate, alkylene diisocyanates of four to 12carbon atoms, inclusive, such as tetramethylene diisocyanate,pentamethylene diisocyanate, hexamethylene diisocyanate, heptamethylenediisocyanate, octamethylene diisocyanate, nonamethylene diisocyanate,decamethylene diisocyanate, dodecamethylene diisocyanate; cyclohexanel,4-diisocyanate, cyclohexane- 1 ,S-diisocyanate, methylcyclohexane-2,4-and 2,6-diisocyanates, 1,3- and l,4-bis(isocyanatomethyl) cyclohexane,diisocyanatodicyclohexylmethane, B-isocyanatoethylphenyl isocyanate,a-isocyanatobenzyl isocyanate, alkyl isocyanates of one to 12 carbonatoms, inclusive, such as methyl isocyanate, ethyl isocyanate, propylisocyanate, isopropyl isocyanate, butyl isocyanate, amyl isocyanate,hexyl isocyanate, heptyl isocyanate, octyl isocyanate, nonyl isocyanate,decyl isocyanate, dodecyl isocyanate; cyclohexyl isocyanate andchlorohexyl isocyanate, and the like.

The novel feature of this invention is inclusion of molecular sieves inthe reaction mixture during phosgenation of the amine or amine salt.Preferably, the molecular sieves employed are first vacuum dried. Theminimum ratio of molecular sieves to reaction mixture is about 1:100 toabout 50:100 by weight. The preferred ratio is one favoring an excess ofmolecular sieves for increased surface contact with the reactants.

The molecular sieves employed in this invention are a class of materialswell-recognized in the art and are defined as crystalline zeolites, bothnatural and synthetic. The natural crystalline zeolites are sodium andcalcium aluminosilicates such as anocite, chabazite, heulandite,notrolite, stilbite and themsonite. (Encyclopedia Of Chem. Technology,Vol. 12, pg. 295, 1954,1nterscience Pub., Inc., N.Y., N.Y.). Thesynthetic crystalline zeolites are readily available commercially in:the formof hydrated silicates of aluminum and an alkali metal such assodium, calcium, potassium or combinations thereof. The syntheticcrystalline zeolites have varying absorption characteristics which havebeen classified according to pore size and structure types. Types 3A,4A, 5A, X and 13X illustrate. types suitable for use in this invention.Type 3A hasthe. smallest pore opening and will admit molecules having adiameter smaller than about 3 angstroms. Type 13X has the largest poreopening and will admit molecules having a diameter of not more than 9angstroms. Type 4A is the preferred size, having less adverse effect onproduct yields, and has a pore opening which will admit molecules havinga diameter smaller than about 4 angstroms (Linde CompanyBulletin,F-9947D dated Jan. 15, 1963). The techniques for preparation ofthese synthetic crystalline zeolite molecular sieves are well-known, anddescriptions of such preparations may be found for example in US. Pats.Nos. 2,882,243; 2,882,242 and 3,037,843; British Pats. Nos. 898,457;986,864; 1,031,278; Netherlands Pat. No. 6,504,679; French Pat. No.1,404,467; Belgian Pat. No. 635,617 and U.S.S.R. Pat. No. 170,912.

In general, the method of preparation of the above crystalline zeolitemolecular sieves comprises mixing varying proportions of sodiumaluminate with silicon dioxide. Both A and X types are prepared in thismanner. By ion exchange, using techniques well-known in. the art, thevarious calcium and potassium compounds are prepared from the sodiumcompound, thereby making. the various pore size structures or types; forexample type 5A is produced from type 4A by exchanging 75% of the sodiumions of type 4A with calcium ions and type 10X is similarly obtainedfrom type 13X by ion exchange of calcium ions for sodium ions.

In carrying out the process of the invention the phosgenation of :theamine is accomplished using wellknown conventional methods; see, forexample, the procedures described by Siefken, Annalen, 562, 75 et. seq,1949. Illustratively, phosgene, either in a gaseous form or in the formof a solution in an inert solvent such 1 monochlorobenzene. is broughtinto contact with the amine previously dissolved in one of the abovementioned solvents. Appropriate mixing procedures are utilized to obtainthe best contact between amine and phosgene. Although the reaction willproceed at room temperature, it generally is accomplished by firstmixing at a low ormoderate temperature, and then treating the slurrywith more phosgene at a higher temperature of from to 200 C. The lengthof time required to complete the reaction is dependent on the reactiontemperature and the particular amine to be phosgenated. Generally, theend point of the reaction is indicated by cessation of the evolution ofhydrogen chloride gas, a reaction by-product. Following completion ofthe reaction, the solvent is stripped together with excess phosgene andresidual hydrogen chloride, using conventional techniques such asevaporation, separation and distillation. When molecular sieves are usedduring phosgenation in accord with this invention,

, they are easily separated from the reaction mixture followingcompletion of the reaction, by decantation and/or filtration.

The following examples describe the manner and process of making andusing the invention and set forth the best mode contemplated by theinventor of carrying.

out the invention, but are not to be construed as limiting. All partsare by weight unless otherwise stated.

EXAMPLE 1 To a 1 liter vessel fitted with a water cooled condenser isadded gms vacuum dried 4A type molecular sieves (Linde Company, seeTechnical bulletin F-9947-D, Molecular sieve products, Linde CompanyDivision of Union Carbide Corporation, Tonawanda, N.Y., Jan. 15, 1963)and 250' grns dry monochlorobenzene.

Phosgene is added at a rate of 0.83 gms/minute over a 30 minute periodof time while maintaining the reactor vessel at a temperature of about-2 to 2 C. While maintaining this temperature, 21 grnsdicyclohexylmethane-4,4'-diamin'e dissolved in 250 grnsmonochlorobenzene is added over a period of 15 minutes. The mixture isheated to a temperature of about C. to about C. for about 3 hours,'andis agitated continually while an excess of phosgene gas is maintained inthe reaction vessel. An indication that the reaction is complete iscessation of hydrogen- After a period of 90 days, the solution ofdicyclohex- I ylmethane 4,4'-diisocyanate still remains clear andcolorless.

EXAMPLE 2 This example serves as a control and shows preparation of theisocyanate without the use of molecular sieves.

Repeating the process described in Example 1 but omitting the molecularsieves in the reaction vessel, a light yellow mixture is obtained. Uponstanding in a capped container for 90 days, the color has continuallydarkened. Vapor phase chromatography indicates a yield of 15.3 gmsdicyclohexylmethane-4,4-diisocyanate (58.4 theory).

EXAMPLE 3 Using the procedure described in Example 1, but substitutingmolecular sieve types 3A, 5A, X, and 13X respectively (Linde, supra) fortype 4A as used therein, a colorlessdicyclohexylmethane-4,4-diisocyanate is obtained in each instance.

EXAMPLE 4 To a 1 liter reaction flask is added 24.4 gms of an 80:20mixture of 2,4-tolylene diamine and 2,6-tolylene diamine in 250 gmsmonochlorobenzene (solvent) and a mixture of 40 gms phosgene in 250 gmsof monochlorobenzene with 100 gms type 4A molecular sieves (Linde,supra). During addition of the phosgene mixture the reaction mixture ismaintained at a temperature of about 0 C. by external cooling means. Theresulting slurry is heated to 133 C. and an atmosphere of phosgene gasis maintained in the reaction flask. In about minutes, the solutionbecomes clear. After an additional minutes, the reaction flask is purgedwith nitrogen gas and cooled to 50 C. The clear solution is an isomermixture of tolylene-2,4-diisocyanate, and tolylene-2,6-diis0cyanatewhich is colored with a slight yellow tint.

A second preparation of tolylene diisocyanate is made using theidentical procedure outlined above, but

without inclusion of molecular sieves. This second preparation oftolylene diisocyanate is a clear solution but of a much darker yellowcolor.

I claim:

1. In the method for producing an organic isocyanate selected from thegroup consisting of alkyl isocyanate of one to 12 carbon atoms,inclusive; aryl isocyanate of six to seven carbon atoms, inclusive;alkylene diisocyanate of four to 12 carbon atoms, inclusive; arylenediisocyanate of six to 12 carbon atoms, inclusive; chlorophenylisocyanate; chlorophenylene diisocyanate; chlorohexyl isocyanate;cyclohexyl isocyanate; cyclohexane diisocyanate; methylcyclohexanediisocyanate; diphenylmethane-4,4'-diisocyanate and mixtures thereofcontaining polyisocyanates of higher functionality;3-methyldiphenylmethane-4,4'-diisocyanate; bis (isocyanatomethyl)cyclohexane; diisocyanatodicyclohexylmethane;[beta-isocyanotoethylphenyl] beta-isocyanatoethylphenyl isocyanate andalpha-isocyanatobenzyl isocyanate; by the reaction of phosgene with amember selected from the group consisting of the corresponding amine andhydrobromide and hydrochloride salts thereof, the improvement comprisingcarrying out the reaction in the presence of a crystalline zeoliticmolecular sieve selected from those having a pore opening ranging insize from that opening which will admit only those molecules having adiameter smaller than about 3 angstroms to that opening which will admitmolecules having a diameter of not more than 9 angstroms; said molecularsieves being present in an amount corresponding to at least 1 part byweight per parts by weight of reaction mixture.

2. The method of claim 1, wherein the molecular sieve has a pore openingwhich will admit only those molecules having a diameter smaller thanabout 4 angstroms.

3. The method of claim 1 wherein the organic isocyanate isdicyclohexylmethane-4,4'-diisocyanate.

4. The method of claim 1 wherein the ratio of molecular sieve toreaction mixture is such that there is an excess of molecular sieves byweight.

2. The method of claim 1, wherein the molecular sieve has a pore openingwhich will admit only those molecules having a diameter smaller thanabout 4 angstroms.
 3. The method of claim 1 wherein the organicisocyanate is dicyclohexylmethane-4,4''-diisocyanate.
 4. The method ofclaim 1 wherein the ratio of molecular sieve to reaction mixture is suchthat there is an excess of molecular sieves by weight.